Chapter 5 Transistor Bias Circuits

Objectives  Discuss the concept of dc biasing of a transistor for linear operation  Analyze voltage-divider bias, base bias, and collector-feedback bias circuits.  Basic troubleshooting for transistor bias circuits

Introduction For the transistor to properly operate it must be biased. There are several methods to establish the DC operating point. We will discuss some of the methods used for biasing transistors.

The DC Operating Point The goal of amplification in most cases is to increase the amplitude of an ac signal without altering it.

The DC Operating Point For a transistor circuit to amplify it must be properly biased with dc voltages. The dc operating point between saturation and cutoff is called the Q-point. The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.

The DC Operating Point Recall that the collector characteristic curves graphically show the relationship of collector current and V CE for different base currents. With the dc load line superimposed across the collector curves for this particular transistor we see that 30 mA of collector current is best for maximum amplification, giving equal amount above and below the Q-point. Note that this is three different scenarios of collector current being viewed simultaneously.

The DC Operating Point With a good Q-point established, let’s look at the effect a superimposed ac voltage has on the circuit. Note the collector current swings do not exceed the limits of operation(saturation and cutoff). However, as you might already know, applying too much ac voltage to the base would result in driving the collector current into saturation or cutoff resulting in a distorted or clipped waveform.

Voltage-Divider Bias Voltage-divider bias is the most widely used type of bias circuit. Only one power supply is needed and voltage-divider bias is more stable(  independent) than other bias types. For this reason it will be the primary focus for study.

Voltage-Divider Bias Apply your knowledge of voltage-dividers to understand how R 1 and R 2 are used to provide the needed voltage to point A(base). The resistance to ground from the base is not significant enough to consider in most cases. Remember, the basic operation of the transistor has not changed.

Voltage-Divider Bias In the case where base to ground resistance(input resistance) is low enough to consider, we can determine it by the simplified equation R IN(base) =  DC R E We can view the voltage at point A of the circuit in two ways, with or without the input resistance(point A to ground) considered.

Voltage-Divider Bias For this circuit we will not take the input resistance into consideration. Essentially we are determining the voltage across R2(V B ) by the proportional method. V B = (R 2 /(R 1 + R 2 ))V CC

Voltage-Divider Bias We now take the known base voltage and subtract V BE to find out what is dropped across R E. Knowing the voltage across R E we can apply Ohm’s law to determine the current in the collector- emitter side of the circuit. Remember the current in the base-emitter circuit is much smaller, so much in fact we can for all practical purposes we say that I E approximately equals I C. I E ≌ I C

Voltage-Divider Bias Although we have used npn transistors for most of this discussion. There is basically no difference in it’s operation with exception to biasing polarities. Analysis for each part of the circuit is no different than npn transistors.

Base Bias We used base bias type circuit for the purpose of simplicity for the initial discussion of transistor circuits. This type of circuit is very unstable since it’s  changes with temperature and collector current. Try Exercise 5.6

Example Assume that RB=240kohm, RC==2.2kohm, Vcc=12 IBQ=(Vcc-VBE)/RB = 47.08microA ICQ=ß IBQ = 2.35mA VCEQ=Vcc-ICQRC=6.83V VB=VBE=0.7V VC? What is Ic(sat)?

Emitter Bias This type of circuit is independent of  making it as stable as the voltage-divider type. The drawback is that it requires two power supplies. Two key equations for analysis of this type of bias circuit are shown below. With these two currents known we can apply Ohm’s law and Kirchoff’s law to solve for the voltages. Try Exercise 5.7 I C ≌ I E and I C = I B *  I B ≌ I E /  ≌ (-V EE -V BE )/(R E + R B /  DC )‏

Collector-Feedback Bias Collector-feedback bias is kept stable with negative feedback, although it is not as stable as voltage-divider or emitter. With increases of I C, less voltage is applied to the base. With less I B, I C comes down as well. The two key formulas are shown below. Try Exercise 5.8 I B = (V C – V BE )/R B I C = (V CC - V BE )/ (R C + R B /  DC ) assuming I C is much bigger than I B

Troubleshooting Shown is a typical voltage divider circuit with correct voltage readings. Knowing these voltages are required before logical troubleshooting can be applied. We will discuss some of the faults and symptoms.

Troubleshooting R1 Open With no bias the transistor is in cutoff. Base voltage goes down to 0V. Collector voltage goes up to 10V(VCC). Emitter voltage goes down to 0V.

Troubleshooting Resistor R E Open: Transistor is in cutoff. Base reading voltage will stay approximately the same. Collector voltage goes up to 10V(V CC ). Emitter voltage will be approximately the base voltage +.7V.

Troubleshooting Base Open Internally: Transistor is in cutoff. Base voltage stays approximately the same. Collector voltage goes up to 10V(V CC ). Emitter voltage goes down to 0V.

Troubleshooting Open BE Junction: Transistor is in cutoff. Base voltage stays approximately the same. Collector voltage goes up to 10V(V CC )‏ Emitter voltage goes down to 0V.

Troubleshooting Open BC Junction: Base voltage goes down to 1.11V because of more base current flow through emitter. Collector voltage goes up to 10V(V CC ). Emitter voltage will drop to.41V because of small current flow from forward biased base-emitter junction.

Troubleshooting RC Open: Base voltage goes down to 1.11V because of more current flow through the emitter. Collector voltage will drop to.41V because of current flow from forward biased collector-base junction. Emitter voltage will drop to.41V because of small current flow from forward biased base-emitter junction.

Troubleshooting R 2 Open: Transistor pushed close to or into saturation. Base voltage goes up slightly to 3.83V because of increased bias. Emitter voltage goes up to 3.13V because of increased current. Collector voltage goes down because of increased conduction of transistor.

Summary  The purpose of biasing is to establish a stable operating point (Q-point).  The Q-point is the best point for operation of a transistor for a given collector current.  The dc load line helps to establish the Q-point for a given collector current.  The linear region of a transistor is the region of operation within saturation and cutoff.

Summary  Voltage-divider bias is most widely used because it is stable and uses only one voltage supply  Base bias is very unstable because it is  dependant.  Emitter bias is stable but require two voltage supplies.  Collector-back is relatively stable when compared to base bias, but not as stable as voltage-divider bias.